The fact that the upstream HN comment has to explain two identical-sounding options that do different things should help you see what's wrong with the current impl. Your steps don't work with the "Open With -> Options" context menu like they point out, but rather only with the "Get Info -> Open With" options. Not the best UX.

For example, I can imagine if there were instead two radio buttons in "Open With": "Always for this file", "Always for this extension". All in one place.

Doesn't work when Xcode is the default app. I use macdown for mathjax files for math.stackexchange.com, and I have to use "open with" to choose macdown every time.